8/8/2022 Jenny Applequist for MRL
The discovery can be expected to lead to innovations in spintronics.
Written by Jenny Applequist for MRL
A multi-institutional team has just published a new way to “read” an antiferromagnet electrically—that is, a new way to determine what its magnetic state is.
The discovery is important because magnets play a foundational role in much of today’s technology. For example, computer memory is generally based on magnets; information is stored in the alignment of magnets’ north and south poles, which signify ones or zeros.
Unfortunately, ordinary magnets suffer from the problem of uncontrollable pole reversals, which can happen spontaneously or in response to heat or light. As devices become smaller and more complicated, such reversals are expected to cause increasing instability.
However, “antiferromagnets”—magnetic materials in which neighboring microscopic magnetic regions are oppositely aligned, meaning that the poles are alternately oriented north-south, south-north, north-south, and so on—are largely free from that problematic flipping, as their configuration tends to lock everything into a stable orientation. For that reason, researchers have been seeking ways to monitor the magnetic orientations in antiferromagnets, and thus be able to use them for applications currently dominated by ordinary magnets.
“The big picture is that we took... an antiferromagnet, and we figured out a new way to differentiate between its states,” says Nadya Mason, who is one of the paper’s authors and also the Ph.D. advisor of co-lead author Julie Soho Shim. “And that new way is by either increasing the magnetic field, or changing an electrical current. So we found a new way to electrically monitor an antiferromagnet.”
Specifically, the team’s experiments showed the presence of “unidirectional magnetoresistance” (UMR)—a change of resistance due to reversal of current direction—in a bilayer of antiferromagnet and non-magnetic metal.
UMR is unique in that the change in resistance depends on the direction of either the magnetization or the electric current. It’s the first time this direction-dependent effect has been observed in antiferromagnets. Further, the team also found that the observed UMR changes direction as larger and larger magnetic fields are applied—a phenomenon that hasn’t been observed in other magnetic systems.
Shim, a Ph.D. candidate in Physics, emphasizes that “the magnitude itself of this effect is pretty small to be utilized practically. But because UMR is enabled just by adding a single material layer [the non-magnetic metal] on top of the antiferromagnetic layer, it can contribute to designing improved antiferromagnetic spintronics devices [that are] more compact and have a greater degree of freedom in the material choice.”
Spintronics leverages not only electrons’ charges, as in conventional electronics, but also the electrons’ spins, particularly their magnetic characteristics.
Why do so? “The benefit of flipping spins versus charges is that any charge motion leads to energy loss,” says Mason, who is the Rosalyn Sussman Yalow Professor in Physics and MRL and the newly named director of the Beckman Institute. “If you don’t move charges around, if you just change the spin state, you can lose less energy, because spins don’t interact with material in the same way and they don’t give off heat in the same way. This is one of the big advantages when we talk about spintronics: it can be faster and more compact, but it can also be much more energy-efficient.”
The new work can advance spintronics by providing a way to monitor the spins in antiferromagnets and thus help build simpler spintronics devices.
Mason reflects, “It was really nice to discover how much the orientation of the magnets mattered here, because it connects to a lot of other materials that have interesting arrangements of charges and spins. Understanding how fundamental atomic arrangements lead to this effect may lead to a greater understanding of the electronic behavior of a much broader class of materials.”
The work was a collaboration with Axel Hoffmann’s group in Materials Science, as well as researchers at Argonne National Lab, Wayne State, and Case Western. The findings highlight the valuable multidisciplinary collaborations taking place through UIUC’s NSF-funded MRSEC center.
The paper is Soho Shim et al., “Unidirectional Magnetoresistance in Antiferromagnet/Heavy-Metal Bilayers,” Physical Review X, vol. 12, article 021069. https://doi.org/10.1103/PhysRevX.12.021069